Powder mixtures having a cellulose ether binder are used in forming articles of various shapes. For example, metal powder mixtures are formed into honeycombs which are used as substrates in catalytic and adsorption applications, (EHCs). The mixtures must be well blended and homogeneous in order for the resulting shaped body to have good integrity in size and shape and uniform physical properties. The mixtures have organic additives in addition to the binders. These additives can be surfactants, lubricants, and dispersants and function as processing aids to enhance wetting thereby producing a uniform batch.
Distortion of the extrudate can occur during (1) flow through the die and at the die exit, (2) cutting, (3) handling, and (4) drying.
Rapid-setting characteristics are important for maintaining the shape of extrudates. The quicker the extrudate can be solidified after forming, the less chance of the dimensions of the greenware being altered in subsequent cutting and handling steps. This is especially true for a fragile thin-walled or complex shapes such as honeycombs or a product having a large frontal area.
Various techniques have been disclosed for rapid stiffening of such batches. For example U.S. Pat. No. 5,258,205 relates to dipping various rapid setting additives e.g. surfactants, binders and water soluble polymers e.g. polyethylene glycol, at elevated temperatures, e.g. around 70.degree. C. This relatively high temperature makes the process undesirable from an operational and economic standpoint. Also, hancling of polyethylene glycol is difficult because it is very tacky. Other techniques, such as lowering of lubricant and/or water content require use of high mixing torques and extrusion pressures. Radio frequency stiffening techniques require a relatively high capital investment such as in-line shielding at the die exit.
Prior methods such as the above described techniques involve extrusion of soft batches and result in stiffening of the extrusion mixture only well after the extrudate has exited the die and not before or immediately after exiting the die.
There still remains a need for a method of earlier stiffening, such as before and instantaneously upon exiting the die to maximize shape retention of extrudates in subsequent handling, especially for complex structures as honeycombs. The present invention fills this need.